By way of general background, a bipolar transistor is a three-terminal device having a base region, a collector region and an emitter region. There is one semiconductor junction between the base and emitter regions, and another junction between the base and collector regions. The collector and emitter regions of a bipolar transistor are doped with impurities that are of the same conductive type, i.e., N-type or P-type, and the base region is of the opposite conductivity type. Thus, a bipolar transistor is said to be an N-P-N or P-N-P type. General theory of operation of bipolar transistors is well known and need not be discussed herein.
For large-scale integrated circuitry, the usual design objectives include high packing density. In particular, many device layouts of transistors are required on typical integrated circuit ("IC") chips to facilitate interconnection of the transistors. For example, it may be desirable for one transistor to have a collector-base-emitter device layout, while a collector-emitter-base device layout may be desirable for a second device on the IC chip. Disadvantageously, transistors with different device layouts typically have different geometries and do not "fit" closely together on a substrate. Thus, the use of a variety of conventional transistors, having different device layouts, typically limits both circuit density and overall wireability.
A further limitation of conventional bipolar transistors is the high-frequency performance of the device. It is well known that there are certain circuit parameters of bipolar transistors that affect the speed of operation and performance at high frequencies. Two of the most significant of these parameters are the base resistance (R.sub.b) and the collector-base capacitance (C.sub.cb). Reduction of either or both of these parameters results in improved speed and high frequency performance. However, in the past, a design having a low R.sub.b typically featured a high C.sub.cb and vice-versa. Thus, a need for a device featuring a low R.sub.b and C.sub.cb exists.
Thus, there remains a need for a bipolar transistor that provides improved high speed performance, while at the same time allowing substantially greater device packing densities and wiring options. The present invention is directed toward solving the above noted problems, and providing such a device.